swiotlb: lift the double initialization protection from xen-swiotlb

Lift the double initialization protection from xen-swiotlb to the core
code to avoid exposing too many swiotlb internals.  Also upgrade the
check to a warning as it should not happen.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>